1. Field of the Invention
The present invention relates to a transmission apparatus for transmitting drive power, suitable for use as a speed change gear accommodated in drive mechanisms of diverse kinds of computer-controlled machines, such as industrial robots.
2. Description of the Related Art
There are known speed change gears, especially speed reduction gears referred to as hypoid speed reduction gears and cyclo speed reduction gears, in which an externally toothed element is rotatably incorporated in an internally toothed element having a number of teeth larger than that of the externally toothed element. The externally toothed element is eccentrically moved in rotational engagement with the internally toothed element. A speed reduction effect is obtained from the externally toothed element due to the difference of the tooth numbers.
In such conventional speed reduction gears, since the externally toothed element from which a rotational output is taken moves eccentrically, an eccentricity absorption mechanism employing an Oldham's coupling or a pin-and-hole engagement mechanism is disposed between the externally toothed element and an output shaft. This kind of eccentricity absorption mechanism suffers from the problem of play appearing in the Oldham's coupling or the pin-and-hole engagement mechanism. In a conventional hypoid speed reduction gear or cyclo speed reduction gear, such play or backlash is rather large. Accordingly, the conventional hypoid or cyclo speed reduction gear is not suitable for use in drive mechanisms using a servo mechanism for positioning the driven element at a high speed and accuracy.
The present inventor previously proposed an improved speed reduction gear (Japanese Unexamined Patent Publication (Kokai) No. 59-170549) in which the externally toothed element is functionally divided into three separate elements, namely, drive roll elements rolling on the internal tooth surface of the internally toothed element, a carrier for carrying the drive roll elements, and a wave generator for providing the drive roll elements with a radial displacement. The improved speed reduction gear thus simultaneously reduces rotational speed and absorbs eccentric movement. However, since the drive roll elements are loosely held in the tooth spaces of the carrier, play appears in the holding portion of the drive roll elements. Also, sliding friction occurs between the drive roll elements and the carrier.